The present invention relates to an optical fiber fabrication method of fabricating an optical fiber with desired optical characteristics by drawing an optical fiber and, more particularly, to a method suitable for fabrication of a dispersion compensating optical fiber.
In an optical transmission system for optical communication with signals of multiple wavelengths propagated in an optical fiber transmission line, in order to restrain waveform deterioration of the signals propagating in the optical fiber transmission line, the absolute value of accumulated chromatic dispersion of the optical fiber transmission line is desirably as small as possible at the signal wavelengths (e.g., at 1.55 xcexcm). However, standard single-mode optical fibers used as optical fiber transmission lines have the zero dispersion wavelength near the wavelength of 1.3 xcexcm and the chromatic dispersion of about 17 ps/nm/km at the wavelength of 1.55 xcexcm. With the optical fiber transmission line constructed of only such a single-mode optical fiber, optical communication is thus infeasible at high bit rates because of the deterioration of signal waveform. Then a dispersion compensating optical fiber with negative chromatic dispersion at the wavelength of 1.55 xcexcm is utilized to compensate for the chromatic dispersion of the singlemode optical fiber, thereby decreasing the absolute value of mean chromatic dispersion at the wavelength of 1.55 xcexcm.
The inventors investigated the conventional optical fiber transmission lines and discovered the following problem. Namely, in order to decrease the absolute value of mean chromatic dispersion of the entire transmission line composed of the single-mode optical fiber and the dispersion compensating optical fiber, it is necessary to properly design a ratio of lengths of the single-mode optical fiber and the dispersion compensating optical fiber, properly design the chromatic dispersion of the dispersion compensating optical fiber according to the chromatic dispersion characteristic of the single-mode optical fiber, and produce the optical fibers as designed.
However, the chromatic dispersion characteristic of the dispersion compensating optical fiber sensitively varies according to variation in glass size (fiber diameter of the dispersion compensating optical fiber). Processing accuracy of the optical fiber preform was insufficient and measurement accuracy of refractive index profile of the optical fiber preform by a preform analyzer was also insufficient. Accordingly, it was difficult to fabricate the dispersion compensating optical fiber having an objective chromatic dispersion characteristic with high accuracy. The above-stated problem is conspicuous in fabrication of the dispersion compensating optical fiber, but also arises in fabrication of optical fibers of other kinds.
The present invention has been accomplished in order to solve the problem as stated above, and an object of the invention is to provide an optical fiber fabrication method by which an optical fiber with an objective chromatic dispersion characteristic can be obtained readily.
An optical fiber fabrication method according to the present invention is a method of fabricating an optical fiber with a desired optical characteristic by controlling an outside diameter of the resultant optical fiber. In this optical fiber fabrication method, a cutoff wavelength is measured in an optical fiber with a fixed length obtained by first drawing a part of an optical fiber preform, a target glass diameter for yielding an objective chromatic dispersion characteristic is determined based on the cutoff wavelength thus measured, and the rest of the optical fiber preform is drawn while an outside diameter is controlled to the target glass diameter thus determined.
Since the optical fiber fabrication method according to the present invention is arranged to feed the measurement result of the cutoff wavelength in the optical fiber with the fixed length obtained immediately after the start of drawing of the optical fiber preform, back to the control of the outside diameter, the optical fiber with the objective chromatic dispersion characteristic can be obtained readily even if the processing accuracy of the optical fiber preform is poor or even if the measurement accuracy of refractive index profile of the optical fiber preform by the preform analyzer is poor.
An optical fiber fabrication method according to the present invention may also be arranged to control the outside diameter by making use of the measurement result of a mode field diameter in the optical fiber with the fixed length obtained after the start of drawing of the optical fiber preform, instead of the cutoff wavelength. In this case, the optical fiber fabrication method is arranged to determine the target glass diameter for yielding the objective chromatic dispersion characteristic, based on the mode field diameter thus measured, and then to drawn the rest of the optical fiber preform while controlling the outside diameter to the target glass diameter thus determined.
By this optical fiber fabrication method, the optical fiber with the objective chromatic dispersion characteristic can be obtained readily even if the processing accuracy of the optical fiber preform is poor or even if the measurement accuracy of refractive index profile of the optical fiber preform by the preform analyzer is poor.
Each of embodiments of the invention will become fully understood from the detailed description and accompanying drawings which will follow. It is to be considered that these embodiments are presented simply for illustration but do not limit the present invention.
The scope of further application of the present invention will become apparent from the detailed description which will follow. It is, however, noted that the detailed description and specific examples present only preferred embodiments of the invention and thus are provided only for illustration and that various modifications and improvements within the spirit and scope of the invention are obvious to those skilled in the art from the detailed description.